Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, February 2004. / Vita. / Includes bibliographical references. / The chemistry of fluorine, F2, and xenon difluoride, XeF2, with clean Si are nearly indistinguishable. Both species react via the atom abstraction mechanism, whereby a surface dangling bond abstracts a F atom from the incident molecule, scattering the complementary F atom or XeF fragment into the gas phase. However, once all dangling bonds are fluorinated and a coverage of 1 ML F is attained, the chemistry of these two species diverges. Further exposure to F2 results in no increase in F coverage, indicating reactions of F2 with the Si-Si -bonds do not occur. In contrast, further exposure to XeF2 results in additional reaction, cleaving the Si-Si bonds and increasing the coverage beyond 1 ML F. Eventually, sufficient fluorination occurs to effect the desorption of the etch product SiF4. The experiments described in this thesis have been designed to probe the origin of the observed difference in reactivity of F2 and XeF2 with a fluorinated Si surface. Two possible explanations are presented that likely account for the difference in reactivity of F2 and XeF2 with fluorinated Si. The first is the reaction of atomic fluorine, which is produced by the gas phase dissociation of XeF following an abstraction event, with the Si. The second is vibrational excitation of the Si lattice, induced by the initial collisions of XeF2, thereby enabling reaction of the F atoms with the Si-Si bonds. The first chapter describes experiments investigating the chemical dynamics of the reaction of a beam of XeF2 with Si(100). The scattered intensities and the time-of-flight distributions of the reaction products XeF2, XeF, and Xe are measured as a function of coverage and scattering angle, as is the intensity of scattered F atoms. / (cont.) The F atoms arise from the gas phase dissociation of XeF, which results from partitioning of the exothermicity of the abstraction reaction of XeF2 with Si into the internal energy of XeF. The time-of-flight and angular distributions of F and Xe are simulated by applying conservation of momentum, energy, and flux principles to the measured XeF time-of-flight and angular distributions using only two fitting parameters: the average internal energy partitioned to XeF and the allowed range of molecular orientations of XeF as it is scattered from the surface. These parameters are adjusted to reproduce the measured F atom TOF spectra at three scattering angles for the coverage range 0 - 0.22 ML F. The same parameters are then used to predict the Xe atom TOF distributions resulting from the dissociation of XeF. The intensity of the TOF distributions for scattered Xe is scaled such that one Xe atom is produced for each F atom in the simulation. The simulated TOF spectra result from the analysis of 8,085,000 initial XeF trajectories, which are computed at eight coverage ranges. The simulation is found to reproduce the scattered Xe TOF spectra well, including the intensity, with no fitting of the simulated Xe spectra to the data. This excellent agreement corroborates the XeF dissociation as a gas phase event. The dissociation of XeF is the first observed case of a gas phase dissociation resulting from partitioning of exothermicity to a product of a gas-surface reaction. In addition, the simulation indicates that approximately 10% of the F atoms resulting from XeF dissociation are directed back toward the surface ... / by Robert Charles Hefty. / Ph.D.
| Identifer | oai:union.ndltd.org:MIT/oai:dspace.mit.edu:1721.1/30113 |
| Date | January 2004 |
| Creators | Hefty, Robert Charles, 1976- |
| Contributors | Sylvia T. Ceyer., Massachusetts Institute of Technology. Dept. of Chemistry., Massachusetts Institute of Technology. Dept. of Chemistry. |
| Publisher | Massachusetts Institute of Technology |
| Source Sets | M.I.T. Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 420 p., 15033050 bytes, 15032847 bytes, application/pdf, application/pdf, application/pdf |
| Rights | M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission., http://dspace.mit.edu/handle/1721.1/7582 |
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